Antimonide coated magnetic materials



2,999,777 ANTIMONEDE COATED MAGNETIC MATERIALS Ernest J. Yamartino and Richard B. Falk, Wakefield,

Mass, assignors to General Electric Company, a corporation of New York This invention relates to improved magnetic materials comprising elongated ultra fine magnetic particles and to the process of manufacture of such materials.

The m'anufacture'of elongated magnetic particles having transverse dimensions which are those of a single magnetic domain by plating a magnetic material into a molten metal cathode is described' in patent'application Serial No. 500,078, filed April 8, 1955, now Patent 2,974,- 104, granted March 7, 1961, assigned to the same assignee as this invention. By maintaining the interface between the electrolyte and the liquid cathode in an undisturbed or quiescent condition, elongated particles of magnetic material having a median elongation ratio of at least 1.5 to 1 and having at least one half of the particles possessed of an elongation ratio of at least 2 to 1 are produced. This represents a significant advance over the prior art which produced largely spheroidal or relatively blunt magnetic particles. In the above process, the magnetic particles are removed from the molten metal cathode such as liquid mercury as by means of a permanent magnet dipped into the cathode, the magnetic material being extracted as a putty-like mass of fine magnetic particles and mercury. The particle-mercury slurry is then heated for a few minutes at about 200 C. to 300 C. and after cooling to room temperature, a trace of a nonmagnetic metal such as tin is added. Gross removal of the magnetic particles from the mercury is carried out by oxidizing the iron with air and the resulting powder is then washed and vacuum or hydrogen baked at a low temperature to eliminate the last traces of mercury. A non-magnetic metal filler or a non-metallic filler is then mixed with the elongated magnetic particles, the particles being aligned by a magnetic field and the mixture pressed into a final firm magnetic structure. Further details of this process will be found in the above referred-to patent application which is hereby incorporated by reference in this application. The magnetic particles as plated have a dendritic structure and the heat treatment above described reduces the branch-like structure and increases the iron particle diameter, thus increasing the coercive force of the iron or other magnetic material substantially. The addition of a material such as tin, zinc, aluminum, manganese, nickel, antimony or other metal which compounds with the magnetic material in the slurry coats the elongated particles separating them with a non-magnetic material. According to the above application, the last traces of mercury can be removed by washing the coated, elongated particles by adding a layer of lead alloy such as lead-antimony alloy to the slurry, thus reducing the mercury concentration and removing the cluster of elongated particles from the slurry with a permanent magnet. These steps of dilution and removal of the magnetic material are repeated until the mercury concentration is at the point desired. The last traces of mercury can also be removed by heating the slurry of mercury and coated elongated magnetic particles at a temperature of about 250 C. for

Patented Sept. 12, 1961 about 3 hours under a vacuum of about 1 micron of mercury.

\Vhile the use of oxidation techniques to remove the mercury or other molten metal matrix results in the eilcctive removal of the iron and produces a magnetic material suitable for many purposes, it is inherently accompanied by substantial losses of magnetic saturation (B residual induction (B and coercive force (H and cor responding losses in the magnetic energy product. The oxidation process using air or oxygen or other chemical methods such as the employment of potassium permanganate or potassium chromate etc. also leaves-up to 30 percent of mercury or molten metal matrix with the oxidized iron. Metal washing techniques for removal of the major part of the mercury, as well as the last traces of mercury using low melting point alloys were explored, but these all'oysare either too expensive forfmass production purposes or have poor physical properties. Vacuum distillation procedures, while successful in removing mercury, are operative only at relatively high temperatures of above about 250 C. and the high temperatures cause'the elon: gated particles to spheroidize, resulting in poor coercive forces and lower magnetic energies in'general.

' From the point ofview of time and ease of operation, Vacuum distillation techniques are much to be preferred over metal washing techniques and the oxidation techniques. However, in order to retain the magnetic properties of the magnetic material at distillation processing temperature above 250 C., it is necessary that the magnetic material particles be stabilized by some means at such temperatures. l

' A principal object of this invention is to provide a means for stabilizing finely divided magnetic particles which will protect them against degradation at relatively high temperatures above 200 C.300 C.

Briefly the invention comprises the provision of an intermetallic layer on the surface of the magnetic particles such layer effectively protecting the particles against the degrading effects of high temperatures. Also encompassed within the invention is the process of producing magnetic particles as described above in which the mercury or other plating matrix or medium is removed by means of distillation without detracting from the magnetic qualities of the magnetic particles.

Those features of the invention which are believed to be novel are set forth in the claims appended hereto. The invention will, however, be better understood and further advantages thereof be appreciated from a consideration of the following description.

It has been found that antimony when reacted with a magnetic particle such as of iron, iron-cobalt, etc. forms an antimonide on the surface of the particles which eflYectively separates the particles one from the other, protects them against chemical attack, prevents spheroidizing or growth of the particles at elevated temperatures and improves the magnetic qualities of the particles. It has further been found that only antimony out of a large number of metals examined provides the necessary thermal stability which was sought. Shown in Table I below are various metals which were used to coat magnetic iron particles along with the stability of particles so coated at various temperatures in the absence of oxidizing conditions.

3 Table 1 Iron Stability at 170 0., 48 Hrs.

Iron at 250 Iron at 400 6., 48 Hrs.

Additive to Iron-Mercury 0., 48 Hrs.

Stable Stable Unsgablenn Stable. Unstable.

. Antimony do do 1 No reaction takes place.

From the above, it is at once evident that only antimony of all the metals shown is suitable as a coating material for magnetic particles as a protective device against degrading effects of high temperatures.

Since antimony is insoluble in mercury, special techniques were developed to insure intimate contact of the antimony with the iron or iron cobalt or other magnetic particles in order to form the requisite coating. This is as contrasted with mercury-soluble metals in which case the coating metal can merely be dissolved in the mercury to attain an atomic dispersion which effectively contacts all magnetic particles within the mercury. Several methods of making the antimony intimately available to coat all magnetic particles in a mercury-magnetic particle slurry or putty have been developed in connection with this invention. One procedure is to combine the mercury-insoluble antimony with a mercury-soluble metal such as lead, zinc, titanium, etc. to form a binary alloy one part of which is still insoluble in mercury while the other part is soluble. Upon addition of the binary alloy to the mercury, the soluble component readily dissolves in the mercury carrying with it the insoluble antimony into intimate contact with the magnetic particles for the reaction forming the antimonide. Alloys of any proportions of antimony to soluble metal can be used. It is convenient to have as much antimony as possible in the alloy consistent with ease of mixing with the slurry. A 50-50 alloy is typical of those which have been found to be useful. Of course, if desired, ternary or other alloys can be used with antimony as the insoluble component. The amount of alloy added to the magnetic particle-mercury slurry depends upon the concentration of magnetic particles therein. Generally for 1,000 grams of a 3.5% by weight iron-mercury slurry from 6 to 12 grams, preferably grams (for highest magnetic energy), of a 50-50 antimony-lead alloy, for example, are suitable. This gives a weight ratio of iron to antimony of from about 5.8 to 11.7. Any other procedure which provides the ratio of iron to antimony is suitable.

Another means of adding antimony 'to the magnetic particle-mercury slurry is to electroplate it into mercury or a mixture of antimony particles and mercury which serves as a cathode. During the electrolytic deposition, the antimony is readily wet by the mercury. The electrolytic antimony in mercury can then be reacted with the magnetic particles. Any concentration of electrode deposited antimony in mercury can be used. Typically an electro-deposited mixture by weight 30% antimony and 70% mercury has been found to be very useful. For 1000 grams of a 3.5% by weight iron-mercury slurry, about 25 to 40 grams, preferably 35 g. (for highest energy), of such a mixture is used for an iron to antimony weight ratio of from about 2.9 to 4.7. Relative- 1y more antimony is required when it is electroplated into mercury because antimony oxides and hydroxides are also formed during this process.

As pointed out above, the antimony reacts with the iron or other magnetic material to form an antimonide or coating on the magnetic particles which renders them resistant to degradation at temperatures up to 400 C. for several days. (Whenever iron is referred to herein as the magnetic material, it will be understood that ironcobalt is also included.) In order to promote the formation of the antimonide, the mixture of the magnetic slurry and the antimony or antimony and carrier such as an alloy is heated to a temperature of about 200 C. for about ten minutes. Temperatures up to about 300 C. are also useful for shorter periods of time, there being a time-temperature relationship.

If the final matrix of the iron particles to form a final magnetic structure is to be an organic material such as a drying oil (tung oil, linseed oil, cottonseed oil, etc.), the heat treatment of the iron-antimony mixture takes place immediately after the addition of the antimony to the iron. On the other hand, if a non-magnetic metallic matrix such as lead is to be used, the latter is added preferably before the heat treatment. The use of lead as a matrix material is more fully set forth and claimed in copending application S.N. 702,803, filed concurrent ly herewith, assigned to the same assignee as this invention. Pure lead provides a matrix material or medium which protects the coated particles from oxidation and from which mercury may be distilled or washed or otherwise treated at relatively high temperatures without reacting with the antimonide coating on the magnetic particles. The lead may be added as elemental lead in the form of chunks or pellets or as mixtures of lead and mercury-a 50-50 mixture of lead and mercury being particularly easy to blend into a uniform mixture with the iron-antimony-mercury containing slurry. The amount of lead is critical only insofar as that a minimum amount of 50 grams of lead per 1000 grams of iron antimony and mercury slurry should be added to protect the iron from oxidation when the mercury is removed as by distillation. Amounts in excess of 50 grams of lead may, of course, be added but these aflfect only the so-called packing factor or magnetic particle concentration of the finished magnetic structure or article.

Directing attention now to the case in which lead is used as a matrixing material and the lead-antimony iron particle slurry has been heat treated, the next step in the procedure is to compress the material in a nonmagnetic compacting mold, die, etc. while subjecting it to a magnetic field, the purpose being to align the elongated iron particles in the direction of the magnetic field in order to obtain the optimum ratio of residual saturation induction of the iron or other magnetic material and to maintain this ratio of B,/ B during the removal of mercury by distillation. Conveniently the pressure used is 3,000 lbs. per square inch or higher, preferably 10,000 p.s.i., and the magnetic field has a strength of 4000 gauss or higher. This procedure also removes some of the mercury.

The mercury is essentially removed from the iron antimony-lead-mercury mixture by vacuum distillation at an elevated temperature. The iron antimonide layer on each particle allowing this operation to be carried out without spheroidizing the magnetic particles and deteriorating their magnetic characteristics. In general, the temperature of distillation is 300 C. to 400 C. the pressure less than one millimeter of mercury and the time of distillation rorn one to four hours. After the vacuum. distillation, the magnetic material contains from about 1% to'3% by weight of residual mercury which it has been found cannot be substantially lowered by various alterations in the operating conditions and may be considered essentially mercury-free.

The final step in the preparation of a finished or complete magnetic structure consists of grinding up the more or less porous mass of iron, antimony. and lead which remains after the vacuum distillation process and pressing it in a directionalizing magnetic field employing conventional powder metallurgy techniques, using typically a pressure of about 50,000 lbs. per sq. inch and a magnetic directionalizing field of about 4,000 gauss or more. Alternatively, the mass remaining after the vacuum distillation process may be pressed hot at a temperature of about 350 C. and at pressures from 3,00050,000 p.s.i. preferably 18,000 p.s.i. to fiow the lead binder into position, at the same time maintaining a directionalizing field of about 4,000 gauss on the material. Shown below in Table II are various characteristics specifically H parallel to the direction of an impressed magnetic field, H perpendicular to the direction of the impressed magnetic field and the B /B ratio for iron treated as above using as the coating material tin, antimony and no coating material at all.

Table II ci ci r/ is Parallel Perpen.

Material as plated 415 400 0. 49 Material after 1st heat treatment 1,130 1, 020 0.75 Material after addition of Coating Materi Sn 1, 570 l, 090 0.85 1, 470 1,090 0.83 1, 130 l, 020 0.75

ing forming step Sn 1, 520 1,090 0. 82 Sb 1, 620 1, 110 0.85 No Coating 1,290 950 0.72 Material after 1 hr. at 350 0. pressure 1 mm. Hg. (Mercury Distillation):

Sn 680 620 0.51 Sb 1, 600 l, 120 0. 85 .No Coating 680 680 0. 50

From the above table it will be noted that the first heat treatment used to remove the dendritic appendages on the elongated particles substantially improves the magnetic characteristics of the material. It will be noted also that after the addition of the coating material and before the heat treatment of the coated material to form the antimonide or other compound, the tin coated particles are substantially equal to or slightly greater in magneticqualities than the antimony treated particles while the material having no particle coating is substantially lower in magnetic characteristics particularly with respect to H (parallel). After a heat treatment of about ten minutes at 200 C. to form the magnetic particle-coating compound, it will be noted that the antimony coated particles have surpassed the tin coated particles in magnetic qualities. After a distillation process of one hour at 350 C. at a pressure of one millimeter of mercury, the tin treated particles have become substantially degraded in magnetic characteristics while those coated with the antimony have remained substantially the same as before the distillation process. It will be noted, in fact, that the tin coated particles have practically the same magnetic characteristics after the distillation step as particles which have not been coated at all.

Shown in Table III below are the magnetic characteristics of materials of this invention pressed into magnetic structures immediately after the heat treatment of the iron antimony combination or before the mercury is distilled off and after the mercury has been distilled off, the pressure being used being 50,000 lbs. per sq. inch and the directionalizing field 4,000 gauss.

From Table III, it will be noted that there is substantially no loss or at most about a 10% loss in magnetic characteristics in the final magnetic structure as prepared fior final use.

As pointed out above, in lieu of lead, other materials such as drying oils and the like may be used as a binder or matrix material for the magnetic particles in forming the final magnetic structure. If drying oils and the like are to be used as the binder material, such materials are not added to the rnagnetic particle material until after the mercury has been distilled therefrom. This is in contra-distinction to the case where lead is used as a binder material in which case the lead binder is added immediately after the antimony addition whereupon the lead acts as a protective material in preventing the oxidation in any form of the coated magnetic particles during the heat treatment or the distillation process. When a drying oil or the like is used as the binder material, the addition of such material to the magnetic particle mixture must be under a vacuum or in the presence of an inert gas such as nitrogen, argon and the like inasmuch as the finely divided particles tend to be pyrophoric.

The following specific example is typical of the practice of the invention using alead matrix material and mercury distillation and the various processing parameters and conditions disclosed above.

To a 3.5% Fe96.5% mercury slurry of fineparticle iron prepared as disclosed in the above-identified copending application Serial No. 500,078, filed April8, 1955, which had been heat-treated for about 10 minutes at C., there was added one-half of one percent by weight of antimony based on the weight of "the slurry, as a 5050' antimony-lead alloy. Next there was added lead as a 50-50 lead-mercury. alloy in the amount of 12% by weight based on the weight ofthe base material. The material was then heated for ten minutes at 200 C. to promote the formation of the antimonide coating on the iron particles. Directionalization and concentration was next carried out by pressing the material in a magnetic field of 4,000 gauss at a pressure of 10,000 p.s.i. reducing the mercury content by about 20%. Essentially all of the rest of the mercury was removed by distilling the material at a pressure of about 1 millimeter of mercury for one hour at 350 C. This reduced the mercury to about 2% by weight of its original amount. The final magnetic structure was produced by pressing the material from the preceding step at a temperature of 350 C. and a pressure of 18,000 p.s.i. under the influence of a 4,000 gauss directionalizing field. The final magnetic structure had a B value of 9,700 gauss, a l3 value of 7,900 gauss, and H value of 560 oersteds and a (BI-I) or maximum energy product value of 2.20 10 gauss-oersteds.

While in the above procedure using a lead matrix the mercury was distilled off because of the ease and short time of processing, it will be realized that the technique of dilution and removal of magnetic material or washing in the well known way as described above and elsewhere can be used if desired. Furthermore, it will be realized that other non-magnetic metallic matrices besides lead may be used, whenever exposure of the particles only to lower temperatures is contemplated and the expense of lower melting alloys is not an important consideration. It will also be appreciated that while magnetic materials having the highest possible magnetic values are obtained when the protective coating described herein along with the described procedure of making magnetic structures thereof are applied to the finely divided single domain particles described above and in copending application Serial No. 500,078, filed April 8, 1955, the teachings of this invention can be applied also to other magnetic particles of various sizes and shapes. While such other magnetc particles will not have the original magnetic; qualities found in the preferred particles, such qualities as they do have will be preserved and even enhanced by the present treatment.

What we claim as new and desire to secure by Letters Paents of the United States is:

1. Finely divided magnetic particles of material selected from the group consisting of iron and iron-cobalt alloy, said particles having thereon a coating comprising the reaction product of antimony and said material, the weight ratio of said material to said antimony ranging from about 2.9 to about 11.7.

2. Finely divided magnetic particles of material selected rom the group consisting of iron and iron-cobalt alloys, said particles being elongated and having a transverse dimension which is that of a single magnetic domain, said particles having thereon a coating comprising the reaction product of antimony and said material, the Weight ratio of said material to said antimony ranging from about 2.9 to about 11.7.

3. A magnetic structure comprising finely divided elongated particles of magnetic material selected from the group consisting of iron and iron-cobalt alloys, said particles having a transverse dimension which is that of a single magnetic domain, said particles having thereon a coating of antimonide, said antimonide being produced by reacting with said material antimony in such amount that the weight ratio of said material to antimony is from about 2.9 to about 11.7.

4. A magnetic structure comprising finely divided magnetic particles of material selected from the group consisting of iron and iron-cobalt alloy, said particles having thereon a coating comprising the reaction product of antimony and said material, the weight ratio of said material to said antimony ranging from about 2.9 to about 11.7.

5. In a magnetic structure comprising finely divided particles of magnetic material selected from the group consisting of iron and iron-cobalt alloy, said particles being elongated and having a transverse dimension which is that of a single magnetic domain, said particles having a matrix of a material selected from the group consisting of lead and lead antimony alloy containing up to about 2% by weight of antimony, a protective coating of antimonide on said magnetic particles, said antimonide being produced by reacting with said material antimony in such amount that the weight ratio of said material to antimony is from about 2.9 to about 11.7.

6. The method of protecting magnetic particles of a material selected from the group consisting of iron and iron-cobalt alloy against magnetic degradation which comprises coating them with an antimonide of said material by reacting with said material antimony in such r 8 amount that the weight ratio of said material to said antimony is from about 2.9 to 11.7.

7. The process of protecting against spheroidization at elevated temperatures in a non-oxidizing medium, finely divided magnetic particles of material selected from the group consiting of iron and iron-cobalt alloys which comprises forming on the surface of said particles an antimonide coatingby reacting with said material antimony in such an amount that the weight ratio of said material to antimony is from abut 2.9 to 11.7.

8. The process of protecting against spheroidization at elevated temperatures in a non-oxidizing medium finely divided elongated magnetic particles having a transverse dimension which is that of a single magnetic domain, said 1 particles being of a material selected from the group consisting of iron and iron-cobalt alloys, which process comprises forming on the surface of said particles an antimonide coating by reacting with said material antimony in such amount that the weight ratio of said material to antimony is from about 2.9 to 11.7.

9. The process of removing mercury at elevated temperatures form a mixture comprising mercury and magnetic particles of material selected from the group consisting of iron and iron-cobalt alloy, which includes the step of protecting said magnetic particles from magnetic degradation during the separation of said particles at elevated temperatures from said mixture which comprises reacting the surfaces of said particles with antimony, said particles being in a non-oxidizing medium, the weight ratio of said material to antimony being about 2.9 to 11.7.

10. The process of removing mercury at elevated temperatures from a mixture comprising (1) mercury, (2) a material selected from the group consisting of lead and 1 lead-antimony alloys containing up to about 2 percent of antimony and (3) elongated particles of a material selected from the group consisting of iron and iron-cobalt alloys which includes the step of protecting said particles against thermal magnetic degradation by forming thereon a coating of antimonide by reacting said material with antimony, the weight ratio of said material to antimony being about 2.9 to 11.7.

References Cited in the file of this patent UNITED STATES PATENTS 2,082,362 Stevens June 1, 1937 2,239,144 Dean et a1. Apr. 22, 1941 2,563,520 Faus Aug. 7, 1951 2,601,212 Polydoroff June 17, 1952 2,825,670 Adams et a1. Mar. 4, 1958 2,849,312 Peterman Aug. 26, 1958 OTHER REFERENCES 

1. FINELY DIVIDED MAGNETIC PARTICLES OF MATERIAL SELECTED FROM THE GROUP CONSISTING OF IRON AND IRON-COBALT ALLOY, SAID PARTICLES HAVING THEREON A COATING COMPRISING THE REACTION PRODUCT OF ANTIMONY AND SAID MATERIAL, THE WEIGHT RATIO OF SAID MATERIAL TO SAID ANTIMONY RANGING FROM ABOUT 2.9 TO ABOUT 11.7. 